Annina Aebischer and Hans U. Güdel
Host sensitized near infrared to visible photon upconversion in 1% Re⁴⁺ doped Cs₂NaYbBr₆
J. Alloys Comp. 374, 60-62 (2004)                 

Abstract: Cs₂NaYbBr₆ doped with 1% Re⁴⁺ was studied by optical absorption and luminescence spectroscopy. In the cubic host lattice the Re⁴⁺ ions substitute for the octahedrally coordinated Yb³⁺. Visible by eye red Re⁴⁺[²T_2g] upconversion (UC) luminescence centered around 14 000 cm⁻¹ is observed upon Yb³⁺[²F_5/2] near infrared (NIR) excitation around 10 805 cm⁻¹ between 10 and 300 K. An energy transfer (ET) step from Yb³⁺[²F_5/2] to Re⁴⁺[²E_g/²T_1g] followed by an energy transfer upconversion (ETU) process on the Re⁴⁺ is shown to be the mechanism responsible for this phenomenon. For Yb³⁺ excitation at 10 805 cm⁻¹ with a power density of 1.3 kW/cm² the VIS/NIR emitted photon ratio is 5% at 10 K.